Northeast of Carlisle, the Kielder Forest area is part of the Border Forest Park just west of the Northumberland National Park. There are two nice mountain bike rides: Deadwater Fell Summit and Kielder Water Loop.
The Kielder Water Loop is an 18 mile trek around the reservoir on firm forest roads with a variety of side trails to keep it interesting. Either start the loop in Kiedler or at the south end from the Tower Knowne Visitor Center, and proceed counterclockwise.
A good starting point is the Tower Knowe Visitor Centre.
Overall, Kielder Water Loop is the 85th most popular mountain bike trail of all 93 mountain biking rides in UK.